Incandescent light for projection apparatus.



E. L. CLARK.

INCANDESCENT LIGHT FOR PROJECTION APPARATUS.

APPLICATION FILED OCT- 17, I912.

Patented Apr. 3, 1917.

INVENTOR EMERSON L. CLARK BY Qd WITNESSES ATTORNEY till EMERSON I4.CLARK, OF LAKEWOOD, OHIO, ASSIGNOR TO NATIONAL CARBON COMPANY, OFCLEVELAND, OHIO, A CORPORATION OF NEW JERSEY.

INCANDESCENT LIGHT FOR PROJECTION APPARATUS.

Specification of Letters Patent.

Application filed October 17, 1912. Serial No. 726,204.

To all whom it may concern:

Be it known that I, EMERSON L. CLARK,

a citizen of the United States, and a resident of Lakewood, in thecounty Cuyahoga, in the State of Ohio, have'invented new and usefulImprovements in Incandescent Light for Projection Apparatus, of whichthe following is a clear and exact description.

My invention relates to lamps for projecting images on screens. Theusual light now in use for projection work, particularly moving pictureapparatus, is the electric arc lamp. In order to use an electric arc toproject the moving picture, for instance, on a screen it is necessary toplace the film at a certain distance away from the arc and to throw thelight on it by means of a condensing lens. It is impractical to placethe film close to the are without any intervening condenser, as thelight from the arc is so irregular that the screen would not be equallylighted.

One of the objects of my invention is to obtain an electric light sourcethat is uniformly distributed over the light emitting area.

Another object is to use in combination with the source of uniformlydistributed light a special projecting lens so that it can be placedcloser to the moving picture film or other like part, and still projectthe desired size of image on the screen.

Another object is to construct a lamp having an incandescent filamentand arranged in such a manner that ample light can be furnished forprojection work.

Other objects will appear in'the appended description.

In the drawings:

Figure 1 is a diagrammatic. view of my improved incandescent projectinglamp shown in position for projecting the picture on a screen, the partsbeing shown in a more or less diagrammatic relation.

Fig. 2 is a representation of another form of my invention shown inposition for projecting the picture on a screen by means of a specialprojecting lens.

Referring to Fig. 1, 1 represents a globular part made of glass or anyother material having a highly polished reflecting surface 2 on theinterior. The drawing shows the lamp as composed somewhat in the samemanner as the usual incandescent lamp having a central rod 3 sealed intothe glass bulb and carrying incandescent wires 4, here shown as metallicfilaments composed preferably of tungsten. It is not absolutelynecessary that the disk 6 be separate from the bulb. If desired asufficient area of the interior may be coated with some material toprevent its being coated with silver when the bulb is being silvered'andthen removing the coat, or the entire bulb may be silvered and part ofit removed for the window. The bulb may have the usual socket 5. At oneside of'the glass bulb a disk (ipervious to light and preferably ofglass is sealed into an opening previously made in the bulb and servesfor what I call the window. However, before the bulb is sealed andpreferably before the disk 6is placed therein, the interior is highlypolished and preferably silvered so as to give a highly reflectingsurface. The idea is to get as efiicient reflecting surface as possible,hence any materials may be used such as silver, nickel, speculum metal,etc., to furnish it. Silver is the most efficient now known andtherefore I prefer to use this as the reflecting surface. After thefilament has been placed in the bulb and the disk 6 sealed in, the airis exhausted from the interior and the tip 7 is sealed. With theexception of the silvered interior of the bulb and the .window 6, theconstruction of this lamp is substantially the same .as the ordinaryincandescent lamp and further description of its mode of construction isnot necessary;

With a source of light surrounded by a reflecting surface and having acertain area of the surface composed of a medium pervious to light, allof the light emitted from the source must eventually proceed throughsuch area provided the efliciency of the reflecting surface is 100% andthe surface is not perfectly smooth. That is, if all the light isreflected back and forth from the reflecting surface and none of it isabsorbed, it must all be emitted through the light pervious area.However, a reflecting surface of approximately'90% efliciency is as highas is at present obtainable, so that with a lamp constructed asdescribed the entire light emitted from the source. will not pass outthroughthe window, but a certain amount will be absorbed.

The concentrating efi'ect produced by my Patented Apr. a, tort.

. If polished silver is used and a window of 1% area provided, therewill be an intensification of l-. 99'. 100 (9O 1) times the intensitywhich would be rodueed were all the light concentrated in a point at thecenter and with no reflecting inclosure. Since this light comes largelyfrom multiple reflections, the light flux within the inclosure issubstantially uniform in all directions, hence the distribution ofintensity about a small light-pervious area will follow approximatelythe cosine law. It will thus be seen that my invention provides forgetting a light source combining the qualities of constant form, fixedposition, uniform illumination, high intensity and simplicity in a highdegree, thus making it suitable for projection purposes.

The preceding law applies specifically to diffused reflection but issubstantially correct for what is commonly termed specular reflection,as it is impossible to obtain an absolutely smooth reflecting surface.Inasmuch as the reflecting surface on the interior of the bulb is madeup of a myriad of minute discontinuous surfaces, substantially all raysproceeding from the light source will eventually emerge through thewindow and the above law will hold.

' Since the light proceeding through this disk is of uniform intensity,the aperture plate 8 may be placed near or against the window 6 so thatthe film or plate 9 can be placed very near to the source of light, orthe aperture plate may be omitted entirely. The usual projection lens 10can then be used to throw the picture on the screen 11.

The size of the projection on the screen 11 is to the size of the ictureon the film 9 as the distance 72 is to t e distance a. In moving pictureapparatus the distance a is usually from 6 to 10 inches, hence with alens 10 as shown in Fig. 1 it will not be possible to place it anynearer than this to the film or plate 9 because the projection on thescreen 11 would then be entirely too large. However, in connection withFig. 2 I have shown a special projection lens which can be placed closeto the film so that there is a greater percent. of the light thrown uponthe screen. Further description of this special projecting lens will begiven later.

It is, of course, preferable to have the reflecting surface on theinterior of the glass bulb, but such surface may be placed on theexterior. With this arrangement, however, there would be a much greaterabsorption of light so that the efiiciency would be diminished.

Since the greater part of the energy from the electric lamp istransformed into heat, the surrounding medium will become ve hot unlessmeans are provided for cooling it down. In Fig. 2 I have shown thesource of light constructed so that it can be properly cooled. In thisfigure the globe consists of two hemi-spherical parts 12 and 13 havingthe Water jackets 14 and 15 therein. These hemi-spherical parts can becast of any metal and the method by which such castings are made is wellknown in the art so that further description need not be given. The twoparts 12 and 13 have their interior surface 16 highly polished andsilvered or coated with any other highly reflecting material. Theincandescent support 17 is cemented or otherwise fastened to the part 12and the terminals' 18 and 19 are roperly sealed by the parts 20 and 21which must be of insulatir g material or else the terminals must beotherwise insulated from the parts 20, 21 in order to prevent a shortcircuit. The disk 22 which serves as the window is cemented or otherwisesecured in the hemi-spherical part 13 as shown and the two parts 12 and13 are securely fastened together to make an airtight joint. The fluidcooling of the metallic hemispherical part prevents the temperature ofthe entire inclosure from increasing to any considerable extent. Sincethe temperature of the glass, which has the lower coefiicient ofexpansion, will be greater than the temperature of the fluidcooledmetal, the difference in the coeflicients of expansion of theglass and the metallic part will thus be compensated. Various otherschemes however, may also be used to secure the desired result. They maybe soldered or joined in any manner, but I have shown them as having theflanges 23, 24 fastened together by means of screws 25. A gasket may beused between the flanges if desired.

After the two hemi-spherical parts have been fastened together asdescribed the air is exhausted from the interior through a tube 26 andthe latter is sealed. If desired a valve could be placed in the tube 26,but it is preferably sealed by heating it until it is plastic enough toweld together. Another way to seal the tube is to coat it on the insidewith a thick coat of solder or brazing material before it is placed inthe part 12.

Then when the air is exhausted the tube 26 can be heated until it isplastic enough to be preventing the lamp from becoming unduly heated.

In order to be able to use theprojecting lens close to the film or plate9, a special lens arrangement is used. This consists of a converginglens 28 and a diverging lens 29 spaced apart so as to have their nodalpoint 0 between the lens and the screen 11. By selecting the rightcurvature for the two lenses and spacing them apart the proper distance,

the nodal point may be spaced the same distance from the screen as thenodal point 0 of Fig. 1 and yet have the projectinglens placed close tothe film 9. -The two lenses 28 and 29 may be appropriately secured in acasing 30.

In Fig. 1 as well as in Fig. 2 the filament supporting means may besilvered to increase the reflecting eificiency.

With my invention it will be seen that suflicient light is obtained froman incandescent source to use it for projection purposes by reason ofthe peculiar construction of the silvered surface and by placing thefilm and the projection lens close up to the source of light. Theadvantages of this arrangement over the are light projection apparatuswill be obvious. After the lamp is placed in po-. sition no furtherattention need be paid to it until it is burned out. Thus the operatingexpenses are reduced. The light obtained from this incandescent sourcehas a uniform intensity and this is admirably suited for projectionpurposes.

My invention is not to be limited to the use of a reflecting surfacearound'the source of light, as I believe I am the first'to use anlit-tilt extended source of light of substantially unvarying intensitythroughout its area, and of substantially the same size as the apertureplate of the picture on the film. This enables me to place the film orplate immediately adjacent the source of light and within what I maycall the limits of the focal plane conjugate to thescreen withoutprojecting any variations due. to the source of light. There is alimiting distance back of the film that an are light or other light ofnon-uniform intensity can be placed and not project a noticeable imageof itself on the screen.

If those lights of non-uniform intensity are placed between this limitand the film they will be projected on the screen in a more or lessdistinct manner depending upon their nearness tothe film. The window inmy device having a light of uniform intensity throughout its entirearea, can be placed near the film and within the said limit, as itsimage is a uniform sheet of light, and hence it produces no unevennessof the projected image of the picture.

Having described my invention what I claim is: j

1. In an incandescent light, a light emitting filament and a bulbsurrounding the filament, the greater part of said bulb 'having a highlyreflecting surface and the remaining part adapted to serve as a Window.

2. In an incandescent light, a light emitting filament and a bulbsurrounding the filament the greater part of the bulb having a highlyreflecting surface on its interior, the remaining part of the bulbserving as a window for the emission of reflected light.

In testimony whereof I have hereunto signed my name.

- EMERSON L. CLARK.

Witnesses:

R. H. HARNEY, A. N. BARRON.

